<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>变量</title>
</head>
<body>
    
</body>
<script>
    var value1 = "123"
    var value2 = 0
    var value3 = null
    var value4;
    console.log(Boolean(value1));//ture
    console.log(Boolean(value2));//false
    console.log(Boolean(value3));//false
    console.log(Boolean(value4));//false
</script>
</html>